The present invention relates to a pneumatic and magnetic bearing type motor in which a dynamic pressure pneumatic bearing is employed as a radial bearing, and a magnetic bearing as a thrust bearing.
An electric motor using pneumatic and magnetic bearings can be operated at much higher speeds than a motor using ball bearings or sliding bearings. Therefore, it is used in technical fields in which high speed rotation is essential. For instance, the motor is employed for the optical deflector of a laser printer, the magnetic cylinder of a video tape recorder (VTR), or a centrifugal separator.
FIG. 3 is a sectional view of an optical deflector for a laser printer which employs a conventional electric motor with pneumatic and magnetic bearings. The optical deflector has a stationary shaft 101, one end portion of which is fixedly secured to a base 103 by shrinkage fitting or press fitting. The base 103 is fixedly coupled to a lower casing 117 with screws 121. Herringbone-like dynamic pressure generating grooves 102 serving as radial bearings are formed on the cylindrical surface of the stationary shaft 101. The radial bearing is used as follows; When a force is applied to a rotary shaft at right angles, the radial bearing prevents the center of rotation from shifting from a predetermined position thereof.
The stationary shaft 101 is inserted into the rotor section of an electric motor with a small gap therebetween. In this case, the motor is to drive the optical deflector. The rotor section comprises: a rotary sleeve 104; and rotary magnets 109 and a balance ring 110 which are all fixedly mounted on the rotary sleeve 104 by press fitting or welding.
The rotary sleeve 104 forming the rotor section has a flange 106, the surface of which is perpendicular to the axis of the sleeve. A polygon mirror 105 is set on the surface of the flange 106, and the flange 107a of a cap 107 is placed on the polygon mirror 105. Under this condition, the polygon mirror 105 and the flange 107a are secured to the flange 106 with screws 108. An air pool 122 is formed between the upper end portion of the stationary shaft 101 and the inner surface of the cap 107.
The stator section of the electric motor comprises: the stationary shaft 101 one end portion of which is secured to the base 103 which is fixedly connected to the lower casing 117 with the screws 121; a stator core 112 secured to the lower casing 117 with screws 116; and stator coils 111 wound on the stator core 112.
A substrate 113 is secured to the stator core 112 with screws 115. A magnetic detecting element 114, which is preferably made up of a Hall element, is mounted on the substrate 113 thus secured. The magnetic detecting element 114 is adapted to detect the magnetic flux of each of the magnets 109 as the rotary sleeve turns.
The magnets 109 are permanent magnets, magnetically attracting the stator core 112. This magnetic attractive force prevents the magnet 109 and the stator core 112 from shifting in the direction of axis of the motor (or in the direction of thrust), that is, it acts to maintain the magnets 109 confronting with the stator core 112. This will be described in more detail. When the magnets 109 are moved upwardly, the magnetic attractive force produces a downward component to pull the rotary sleeve 104 downwardly; whereas when the magnets 109 are moved downwardly, the magnetic attractive force produces an upward component to pull the rotary sleeve 104 upwardly. Thus, the magnets 109 and the stator core 112 are held confronted with each other at a predetermined level by the magnetic attractive force. That is, the magnets 109 and the stator core 112 form a magnetic thrust bearing.
The magnetic detecting element 114 is made up of a Hall element for instance. When the magnets are turned, the magnetic detecting element 114 detects the leakage flux of each of the magnets 119, thereby to detect whether the N pole has passed or whether the S pole, and outputs a detection signal. The detection signal thus outputted is applied to a control circuit section (not shown) through a circuit printed on the substrate 113. In the control circuit section, the detection signal is utilized to determine the directions of currents applied to the stator coils 111, which are wound on the stator core 112. The interaction between the currents and the magnets 109 provides a force to maintain the rotation.
When, in the optical deflector of FIG. 3, the rotary sleeve 104 is turned, the dynamic pressure generating grooves 102 form air layers with high pressure around the stationary shaft 101. The air layers thus formed act to support the rotary sleeve 104 in such a manner that the latter 104 floats above the stationary shaft 101; that is, a so-called "dynamic pressure pneumatic bearing" is provided there. In the motor, the dynamic pressure generating grooves 102 are formed on the cylindrical surface of the stationary shaft 101; however, they may be formed on the inner cylindrical subrace of the rotary sleeve 104. The air layers act to maintain the center of rotation of the rotor section unchanged. For instance when the rotary sleeve 104 is shifted to the left in FIG. 3, then the gap on the right side is increased, and the pressure therein is therefore decreased; whereas the gap on the left side is decreased, and the pressure therein is therefore increased. Thus, the rotary sleeve is pushed to the right side by the difference between those pressures, and finally it is located at the original position.
An upper casing 118 is set on the lower casing 117. The side wall of the upper case 118 has an opening 119 which confronts with the polygon mirror 105. A window glass plate 120 is fitted in the opening 119.
The optical deflector thus constructed operates as follows: When a light beam outputted, for instance, by a laser (not shown) is applied through the window glass plate 120 of the opening 119 of the upper casing 118 to a mirror surface 151 of the polygon mirror 105, where it is reflected. The light beam thus reflected advances through the window glass plate 120 towards a photo-sensitive element. When the light beam emerges from the polygon mirror being reflected by the mirror surface 151, the direction of emergence of the light beam is gradually changed because the polygon mirror 105 is turned, so that the light beam scans the photo-sensitive element in the main scanning direction. When the polygon mirror is further turned, the light beam is reflected by the following mirror surface 151 in the same way. Thus, the light beam scans the photosensitive element over a predetermined angular range. The scanning speed depends on the speed of rotation of the polygon mirror 105.
An example of the above-described optical deflector has been disclosed, for instance, by Japanese Patent Application (OPI) No. 17023/1984 (the term "OPI" as used herein means an "unexamined application").
The specific feature of rotor supporting means in the optical deflector resides in that a journal pneumatic bearing (or radial bearing) provided inside a cylinder, and a thrust magnetic bearing provided outside the cylinder are so arranged that they are partially overlapped with each other in the axial direction. However, this arrangement is disadvantageous in the following points: That is, the unbalance of the thrust magnetic bearing, attributing to the manufacturing accuracy of the components or the assembling accuracy thereof, affects only one side of the journal bearing. As a result, vibrations are caused during rotation, and the bearings are greatly damaged when the rotation is started or stopped, so that the bearings are greatly shortened in service life.
In order to eliminate the above-described difficulties, Japanese Utility Model Application (OPI) No. 70532/1978 has disclosed a technique that magnetic bearings are provided at the upper and lower ends of a hollow rotary shaft.
However, the technique still suffers from the following problems:
(1) It needs at least four magnets which are expensive.
(2) In order to make the upper and lower magnetic bearings equal in performance, it is necessary to physically accurately position them. For this purpose, it is necessary machine the magnets, the hollow rotary shaft, and the stationary shaft with high accuracy.
(3) In order to unify the magnets in the, force of magnetization and in the pattern of magnetization, it is necessary to give 100% inspection to the magnets for flux density, or select magnets acceptable in flux density.
(3) It includes a number of components high in accuracy, and therefore assembling them takes a lot of time and labor.